1. Field of the Invention
The present invention relates to a video signal reproducing apparatus and more particularly to a video signal reproducing apparatus capable of automatically carying out a tracking control.
2. Description of the Prior Art
The present invention is described with reference to a magnetic recording and reproducing apparatus (hereinafter called a VTR) for recording a video signal on a magnetic tape.
Recent progress in high density technology has resulted in development of a VTR with narrowed track pitch. This enables long time recording and reproduction. When reproducing from tape on which the information is recorded with a narrow track pitch of this type, it is important that a reproducing head traces the track on the recorded tape correctly. Several tracking control methods have been proposed and some of them have been put into practice.
According to the conventional tracking control method a control signal (hereinafter called CTL) synchronized with a vertical synchronizing signal is recorded on a track exclusive for the CTL and separate from the video track. At the time of reproduction, the tracking control is carried out by means of the above CTL.
Quite recently, more interest has been aroused by a method which involves recording four different frequency signals (hereinafter called 4f pilot signals) on the four adjacent video tracks superposed on a video signal for one field to be recorded each on one track. At the time of reproduction a tracking control signal is obtained by the difference of reproduction levels of the 4f pilot signals from the two adjacent tracks of the video tracks to be reproduced so as to make the both reproduction levels equal. According to this method no fixed head is needed, nor is a track needed exclusive for the tracking signal. Further, the frequency characteristic of the thus obtained tracking control signal (hereinafter called ATF signal) is sufficient enough to control even tracking errors of relatively higher frequency components.
However, the ATF signal obtained from the above tracking control method using the 4f pilot signals is apt to cause a step shaped level fluctuation for each period corresponding to each field of the video signal (change-over cycle of the reproduction track). This arises because of the fluctuation of the reproduction levels of the 4f pilot signals due to the frequency characteristic of the reproducing head. Further, fluctuation of the recording level of the 4f pilot signals and also fluctuation of the 4f pilot signals takes place due to the tape used. Further, when a plurality of reproducing heads are used a step shaped level fluctuation takes place in the ATF signal owing to the fluctuation of the widths of the reproducing heads, the fluctuation of the mounting positions of the reproducing heads and so on.
Normally, carrying out tracking control at the time of reproduction involves a method for controlling the rotation phase of a capstan shaft with respect to a standard rotation phase of the rotary reproducing head. The frequency response characteristic of the capstan motor to be controlled is generally lower than several HZ. Hence the level fluctuation taking place for each field of the above ATF signal cannot be followed and the phase is controlled with the mean value.
Consequently, in the case of the helical scan VTR in which two reproducing heads are fixed on the rotary drum the tracking adjustment cannot be carried out for each reproducing head so that the influence of the noise component of the ATF signal ocurring from heads fixed diametrically causes mistracking. Further, level fluctuation of the reproduced 4f pilot signals also means level fluctuation of the amount of cross talk from the both adjacent tracks. Hence the difference between the both cross talks is used as the ATF signal so that the level fluctuation influences the tracking error detection sensitivity of the ATF signal and thus the gain of the capstan phase control loop fluctuates. This often destroys the stability of the tracking control.